Recently, with the tendencies of miniaturization and high speed of electronic devices and combination of various functions, signal transmission speed inside electronic devices or signal transmission speed outside of electronic devices is increasing. Thus, a printed circuit board using an insulator having lower permittivity and dielectric loss factor than the existing insulator is required.
As a reflection of such tendencies, recently, there has been an attempt to apply a liquid crystalline polymer (LCP), which is an insulator having lower permittivity and that is less influenced by moisture absorption than the existing polyimide, even in a flexible printed circuit board. However, even if the LCP is applied, permittivity (DK=2.9) of LCP is not significantly different from polyimide (Dk=3.2), and thus the degree of improvement according to the application is insignificant. Further, heat resistance of the LCP is so low so as to cause problems in a soldering process, and compatibility with a PCB manufacturing process using the existing polyimide is lowered in via hole processing using a laser.
Thus, as a solution thereto, there has been an attempt to lower permittivity of a polyimide that is used as an insulator of the existing flexible circuit board. For example, according to U.S. Pat. No. 4,816,516, a polyimide and a fluorinated polymer are mixed to make a molded product. However, this patent is not related to a product for an electronic device requiring low permittivity, but is related to a molded product, and practically uses a polyimide having a high coefficient of thermal expansion and a low glass transition temperature. In addition, a polyimide resin should be processed in the form of a thin film so as to be used in a printed circuit board, but there is no description for a copper clad laminate prepared in the form of a thin film in the above U.S. patent.
U.S. Pat. No. 7,026,032 discloses a method of lowering permittivity of a product that is prepared by dispersing a micro-powder of a fluorinated polymer in a polyimide. The above U.S. Patent describes that a micro-powder of a fluorinated polymer is distributed more in the outer surface compared to the internal core of the insulator. However, as described in the above U.S. patent, since the content of the fluorinated polymer is high in the outermost layer of the insulator, moisture permeation and absorption are lowered due to the fluorinated polymer of the outer surface, thus lowering the total moisture absorption rate, but problems that a flexible copper clad laminate consisting of the existing polyimide did not have may be generated. For example, the polyimide resin described in the above U.S. patent may have lowered adhesion to a coverlay or prepreg and lowered adhesion to an anisotropic conductive film (ACF), the coefficient of thermal expansion (CTE) of the polyimide resin described in the above U.S. patent is too large to be applied for a flexible copper clad laminate, and a fluorinated resin exists in an excessive amount on the surface of the polyimide resin, and thus a fluorinated resin may be melted at a temperature around 380° C. that is applied in a receiving process during a PCB manufacturing process, and there is a danger of delamination of a copper clad circuit from an insulator.
Accordingly, there is a need for development of a material that exhibits low permittivity and yet has a low coefficient of thermal expansion, high elasticity, and a low moisture absorption rate, so as to prepare a low-permittivity printed circuit board.